Drying equipment



Feb. 8, 1938. w. T. ANDERSON ET AL 2,107,275

DRYING EQUIPMENT Filed July 2a, 1956 Wi///'c/m 7. Anderson James M.Henry INVENTORS BY W C; 7%

ATTORNEY.

. an arrangement ?atented Feb. 8, 1938 UNITED STATES PATENT OFFICEDRYING EQUIPMENT William T. Anderson,

Henry, Newburgh,

vPont. de Nemours & Company,

Eggertsville, and James M. N. Y.,' assignors to E. 1. dm

"Wilmington,

3 Claims.

This invention relates to a method and apparatus for drying coated sheetmaterial at a high speed.

The coating of sheet material with cellulose derivatives asconventionally practiced is effected while the sheet unwinds from asupply roll and moves in a horizontal direction. In such a methd, thecloth or other sheet material passes over two transverse rollers whichare in a horizontal plane and the doctor blade bears against thematerial to form a trough between the two transverse rollers. Thecoating composition is continuously poured into the trough on theforward side of the doctor blade. As the fabric passes under the blade,a thin film of the composition being applied is deposited on the sheetand thereafter it is passed into a drying chamber. The conventionaldrying chamber consists of a tunnel which is heated by means of steamcoils or other suitable means located within the tunnel either above orbelow the cloth. The volatile components of the coating composition aredriven OE and drawn by suction fans through suitable ducts anddischarged into the atmosphere or delivered to a solvent recoveryapparatus. Such a drier evaporates the solvent from the fabric coatingslowly; and the speed at which the sheet material is passed through theapparatus is therefore limited. There are also limits which determinethe maximum temperature practical to use preventing increase in dryingrates by this means compared to conventional rates. If on the other handit is desired to pass the fabric through the drying chamber at acomparatively high rate, it is necessary to increase the length of thedrying apparatus which increases the cost of the apparatus.

This invention has as an object the provision of a method and apparatusfor drying coated sheet material rapidly which allows the coating,

to be carried out at higher speeds than heretofore.

Another object of this invention is the provision of a method of dryingfabrics coated with cellulose derivative compositions in which thedanger of explosion is reduced to a minimum.

Another object is the provision of a method which results in amore'eiiective' heatv transfer in drying the coated fabric. Otherobjects will appear hereinafter. These objects are accomplishedaccording to the present invention by of the parts of a drying tunnel inwhich air is circulated at high velocity partly in a countercurrent flowto the coated fabric and partly in a direct flow with the fabric.

In the drawing; the figure represents a diagrammatic sketch of therelation of the parts of the apparatus. The fabric or other sheetmaterial which has been coated is represented as I, the fans whichcirculate air are designated as 2 and the heating means are shown as 3.An adjustable orifice through which the cloth passes and through whichair is drawn into the system is designated as 4. The tunnel in which thevapors are volatilized is shown as 5. An exit pipe leading to theatmosphere or solvent recovery apparatus is shown as 6. The exit orificecorresponding and similar to the intake orifice d is shown as I.

In the operation of the-apparatus, the coated fabric enters theapparatusat t and a current of air is drawn in at the mouth of the tunnel wherethe cloth enters also at 4. The fan circulates the air from this pointthrough the heater 3 back into the tunnel at a point about V; betweenits entrance and exit. At the same time the other fan 2 draws air intothe orifice I and circulates it through the heater back into the tunnel5 at a point approximately from the exit to the entrance of the tunnel.This air is lean in solvent vapors and the amount of the air drawn intothe orifice l is sufiicient to dilute the vapors well below the point atwhich there is any danger of explosion. Part of the lean mixture is ledto the fan which circulates air at the entrance end of the tunnel. Thismixture and the mixture drawn 'from the entrance of the tunnel isinjected into the tunnel at a point about of the distance between theentrance and the exit of the tunnel 5. Part of this mix: ture iswithdrawn in pipe 6 as mentioned above. The air is circulated at arelatively high speed, as indicated in the drawing about 700 cu. ft. perminute is drawn in at l and about 600 cu. ft. per minute is drawn in atl. The high velocity of the air in the tunnel is accompanied by suchturbulence that it is brought into very intimate contact with the wetfilm of composition on the coated sheet material.

As illustrated in the drawing, the air is returned to the tunnel at apoint approximately of the distance from the entrance to the exit. Thisdivision is made at a point at which evaporation has ceased to be thecontrolling factor in drying and diffusion has become the controllingfactor. It has been determined that approximately '70 to 80% of thetotal solvent removed from a. conventional nitrocellulose composition isdrawn ofi in the first third of the drying chamber as illustrated in thedrawing. Removal of the remaining solvents requires approximately twicethe period as required for the-first 70 to 80%. It will be obvious thatthis division is somewhat dependent upon the volatility and vaporpressure of the solvents contained in the cellulose derivative coatingcomposition and'upon other factors governing respective rates of dryingin the two sections of the machine.

It has also been found desirable to circulate air at a highertemperature in the first section than in the second section since theevaporation of the solvent absorbs a considerable amount of heat. Atemperature of 200 F. in the first section and 180 F. in the latter ofthe tunnel has been found to be satisfactory. The film of coatingcomposition on the fabric after it passes through the machine becomeshardened to some extent on the surface and subsequent removal of thesolvent depends upon its diffusion through the film to the surface. Bycirculating air at a temperature of approximately 20 F. lower in thelatter portion of the tunnel it has been found possible to eliminateundesirable exuding of the softener in the coating composition to thesurface film. It has also been found desirable to circulate the warm airin a eountercurrent direction during the first third of the travel ofthe cloth throughthe tunnel and in the same direction as the travel ofthe cloth through the latter $5 of the travel of the cloth through thetunnel.

In the operation of the machine optimum drying conditions are maintainedwhen approximately 8500 cu. ft. of air per minute are circulated by eachfan. A circulation of this order produces a maximum of turbulence with aminimum of resistance to the flow of air in the tunnel. These conditionsrender the process extremely efllcient in carrying out the dryingoperation. In

.fact, under optimum conditions cloth can be dried at a rate as high asyds. per minute.

The fans, heaters, and solvent recovery apparatus maybe of any type anddo not constitute in themselves a part of this invention. The coatinghead may likewise be of any conventional type. However, particularlygood results are obtained when the fabric is coated by means of theapparatus disclosed in the copending application of W. T. Anderson,filed Nov. 24, 1934, Serial No. 754,595.

The'equipment disclosed herein is adapted for drying coated sheetmaterials such as woven or felted fabrics of various types includingcloth, paper, etc., particularly where it is desired to remove thevolatile constituents of the coating composition rapidly. The apparatusis particularly useful in removing explosive or otherwise dangeroussolvents at a rapid rate and without any escape of the solvents into theatmosphere.

The advantages of this machine over the conventional types of coatingmachines are that it can be safely operated at a high speed thusincreasing the efiiciency of the coating operation and reducing the costper-unit of coated material. Another advantage is that when used inconjunction with the coating head of. Anderson mentioned above, there isless tension on the warp threads while passing through the dryer. Thisreduced tension causes little or no pull down in width of thecloth'thereby making it possible to use cloth of a narrower width thanrequired by machines in which the tension through the drier is greater.This reduction in tension also makes it possible to reduce the so-calledrib" effect which is an imprint of the goods showing through the coatingand is particularly objectionable in sateens.

A further advantage is that the dryingchamber is divided into twosections. This makes it possible to maintain a different temperature inthe sections of the drying tunnel than has heretofore been possible in aone section drier. A still further advantage is that approximately halfof the total air taken into the apparatus through the back section isdelivered while lean in solvents to the front section. This assuresproper dilution of solvent regardless of whether the larger portion ofthe evaporation takes place in the front or rear section of the dryingchamber.

As many apparently widely different embodiments of. the invention may bemade without departing from its spirit and scope, it is to be 1. Anapparatus for drying coated fabrics, which comprises a long tunnel,means for passing the coated fabric through the said tunnel,

means for heating air and means for circulating 1 the hot air throughthe first portion of the tunnel in a direction countercurrent to thetravel of the 1 coated fabric, and means for passing hot air in adirection concurrent with the travel of the cloth in the latter sectionof the tunnel and means for passing a portion of. the air circulating inthe latter portion of the tunnel to the air circulating in the firstportion of the tunnel.

2. An apparatus for drying coated fabrics which comprises a long tunnel,means for passing the coated fabric through the said tunnel, means forheating air and means for circulating the hot air through the firstportion of the. tunnel in a direction countercurrent to the travel ofthe coated fabric, and means for passing hot air in a directionconcurrent with the travel of the cloth in the latter section of thetunnel and means for passing a portion of the air circulating in thelatter portion of the tunnel to the air circulating in the first portionof the tunnel, and means for withdrawing a portion of the aircirculating in the first part of the tunnel, and means for supplyingfresh air to the system located at the entrance and exit of the tunnel.

3. Process of removing volatile organic solvents from fabrics coatedwith a cellulose derivative composition containing the said solventswhich comprises passing the said fabric through a long chamber, passingheated air at a high rate of speed over and about'the said fabric in adirection countercurrent to the travel of the said fabric for about $4,of its passage through the said chamber and passing a similar current ofair over the said fabric in a direction the same as the travel of thefabric through the latter portion of the chamber, removing a portion ofthe air from the latter current, and supplying it to the first current,and withdrawing air from the said first current.

WILLIAM T. ANDERSON.

- James M. HENRY.

